1. Field of the Invention
The invention relates to a process for producing high molecular weight polyesters, such polyethylene terephthalate, polybutylene terephthalate, as well as their copolymers, as well as to an apparatus for performing the process. More specifically, the invention relates to a process for producing high molecular weight polyesters in which a pumpable material is produced from terephthalic acid and an alkaline diol esterefied in a multi-stage reactor arrangement in which waste heat is used to produce the vacuum in at least one vacuum reactor.
2. Description of the Prior Art
A known process for the continuous production of polyethylene terephthalate (PETP) based on terephthalic acid and ethylene glycol will be described relative to the apparatus according to FIG. 1. Terephthalic acid and ethylene glycol are introduced into a mixing device 1 in a ratio suitable for esterification, pumped by means of a pump 2 into the esterification reactor 3 and converted at 80 to 90% into bis (2-hydroxyethyl)-terephthalate at temperatures between 240.degree. and 280.degree. C. and pressure between 0.1 and 8 bar. The water obtained from the kinetics of the reaction in the form of steam is passed together with the liberated glycol vapour into the separating column 4, the purified water vapour being drawn off in the upper part of column 4 and condensed in a condenser 6. The condensate is collected in a container 7 and supplied via a pump again, partly as a return flow or reflux, to the separating column 4, whilst the reaction water drains out of container 7. The reaction product passes into a further reactor 8, in which the esterification reaction is largely concluded. Reaction water vapour and liberated glycol are totally condensed and supplied to a recovery means. The esterification product passes into a first vacuum reactor 9 constructed as a degassing vessel, which contains heating coils to compensate the enthalpy loss of the precondensate resulting from the evaporation of the glycol, the oligomers, and the remaining water formed during expansion. Precondensation is continued in a second vacuum reactor 10, whose stirring cascade favourably influences polymer formation and therefore, reaction glycol expulsion.
Final condensation takes place in a third vacuum reactor 11, which can be constructed as a rotating disk reactor. The finished polyester melt is removed by a pump 12 at the reactor outlet and is supplied to a spinning means. With each vacuum reactor 9, 10, 11 is associated a vacuum unit 5, 27, 14 for producing the different vacua in the range 100 to 0.1 mbar. Jet pumps 12, 13 of vacuum units 5, 27 are, e.g., constructed as ethylene glycol - liquid pumps, the glycol 15 leaving jet pumps 12, 13 to flow in the circuit and from time to time being worked up again in a recovery means. Between the vacuum reactors 9, 10, 11 and jet pumps 12, 13, 14 is provided a spray condenser 16, in which the glycol vapours and oligomers from the particular vacuum reactor are deposited. The cooling fluid is glycol, which is injected by means of nozzles. The deposited glycol is also supplied to a collecting container. The vacuum unit 14 producing the vacuum in the final condensation reactor 11 is constructed in a multistage manner; i.e. jet pump 17 is connected to spray condenser 16, which can also be in multistage form and which is preferably constructed as a steam jet pump, which 17a is connected to a direct contact condenser 18, whose outlet issues into a second jet pump 19, which once again leads into a second direct contact condenser 20, to whose outlet is connected a third jet pump 21. Jet pumps 17, 19, 21 are supplied with live steam 22, whilst contact condensers 18, 20 operate with cooling water 23, which is partly obtained from the condensate of condensers 18, 20 and jet pump 21, but partly has to be supplied as fresh water to avoid concentration in the circuit. The condensate is collected in container 24 and is supplied by means of a pump 25 to a heat exchanger 26 or the like. The overflow from barometric container 24, enriched with glycol, aldehydes and other decomposition products, passes into the waste water system.
It is alo possible to use other means as vacuum units, e.g. mechanical vacuum pump means with rotary pump or glycol vapour and liquid jet pumps, provided that they take account the specific requirements of the product it is also possible to correctly construct the upstream devices for the condensation and separation of the reaction vapours and oligomers.
The known processes are inter alia characterized by a relatively poor thermal efficiency of approximately 30%. In addition, in the known processes, undesired side flows occur to a considerable extent and lead to waste water pollution or increased contamination of the recovery means or waste water treatment. In addition, all the known vacuum means must be operated from the outside by additional energy sources and involve high maintenance and capital costs.
The problem of the invention is to improve the known processes for the production of high molecular weight polyesters, in that the overall efficiency is increased, i.e. the plant is operated economically through reduced use of primary energy. In addition, waste water pollution is to be minimized and operational reliability improved. This problem is inventively solved by the characterising features of the main claim.